Development of electronic technologies in recent years is remarkable, and in the fields of hydraulic driven machines, electronic/hydraulic control has been replacing the mechanical control. The electronic/hydraulic control of this type has a high degree of freedom and is inexpensive even when the control is a complicated one. Also, the electronic/hydraulic control is capable of controlling the hydraulic driven machine by the teaching/play back operation. FIG. 4 shows an example of such a control device. When a manual controller 24 is operated, it outputs an operation signal to a control device 23, which is turn operates an electrical/hydraulic valve 22 in accordance with the operation signal so as to couple a hydraulic cylinder 21 to a hydraulic source and thereby operate the hydraulic cylinder 21 by means of a pressure oil supplied from the hydraulic source. When the play back operation of the hydraulic cylinder 21 is to be performed, a predetermined play back instruction signal is given to the control device 23 to call the operation signal stored in a memory device 25 to the control device 23. The control device 23 operates the electrical/hydraulic valve 22 in accordance with that operation signal.
However, the conventional teaching/play back control device of the above-described type has a drawback in that a precise play back operation cannot be performed when the conditions for the teaching operation differ from those for the play back operation, particularly, when the conditions are changed in such a way that a higher output is required during the play back operation. For example, in the case of a teaching/play back control device which is applied to a power shovel, if the soil dug during the play back operation is harder than that dug during the teaching operation, the discharge rate of a hydraulic pump may decrease so as to maintain the power of the hydraulic pump to a substantially fixed value. This may decrease the machine operation speed and generate errors in a subsequent operation because in this teaching/play back system the play back control is performed on a time basis, preventing precise operation from being conducted. Also, in the above-described conventional technique, when the errors of the teaching mode are noticeable, complicated operations are required, including switching over of the operation mode to the normal operation mode, manual correction, and switching over of the operation mode to the play back mode again. Also, in a case where a failure occurs in an electric system of this teaching/play back system, it is impossible to operate the hydraulic cylinder, causing cessation of the operation or deviation of the teaching operation from the play back operation due to the errors in the control device.